This invention relates to an electronic identification system comprising a plurality of transponders, and an interrogator for interrogating the transponders to obtain information stored in the transponders. The invention also relates to a transponder for use in such an identification system. The invention is particularly, although not exclusively, concerned with an electronic identification system using radio frequency (RF) technology for communication between the interrogator and the transponders.
Such an electronic identification system is described for example in European Patent Applications EP 0494114, EP 0585132 and EP 0598624, and in PCT Patent Application WO 93/17404.
Such systems have many potential applications, such as tagging goods, mail, vehicles or animals. For example, such a system could be used in a retail store to identify goods presented by a customer at a checkout. The transponders could be incorporated in tags attached to the goods or to their packaging.
In such a retail application, in order to detect unauthorised removal of goods from the store, an anti-theft gate may be positioned at the store exit, to detect the presence of transponders. It has been proposed in the above mentioned EP 0585132 to disable a transponder for a predetermined period of time (e.g. 10 minutes) after it has been successfully read at the checkout. This ensures that goods that have been paid for do not trigger the anti-theft gate, provided the customer leaves the store within the predetermined period. However, a problem with this is that if the customer later brings the purchased goods back into the store with the transponder still attached, the anti-theft gate will be triggered.
The above-mentioned WO 93/17404 proposes to write information into the transponder at the checkout, to indicate whether the article has been paid for or its removal has otherwise been authorised. This information can then be read at the anti-theft gate, so as to ensure that the anti-theft gate is not triggered by articles that are being legitimately removed from the store, or subsequently brought back into the store. However, a problem with this proposal is that it requires a relatively sophisticated anti-theft gate, capable not only of detecting the presence of a transponder, but also of interrogating it to read the information indicating whether the article has been paid for.
One object of the invention is to provide a novel electronic identification system which is capable of overcoming these problems.
According to the invention there is provided an identification system comprising at least one interrogator and a plurality of transponders, wherein each of said transponders comprises:
(a) means for responding to an interrogation signal from the interrogator by returning a response signal to the interrogator;
(b) means for switching from a first mode to a second mode in response to a mode switching signal from the interrogator; and
(c) level setting means for setting said response signal to a first power level when the transponder is in said first mode, and to a second, lower power level when the transponder is in said second mode.